Process for improving the drainage of cellulosic pulps by adding carbon dioxide directly into the pulp

ABSTRACT

The invention relates to a process for improving the drainage or dewatering of cellulosic pulp suspensions in a paper making machine, a drying machine or the like assembly. The invention also relates to a process for producing a dried cellulosic product from said pulp suspension. The drainage of said pulp suspension is improved by treating it with carbon dioxide just prior to its entering a dewatering device such as the wire section of a paper machine.

The present invention relates to a process for improving the drainage ordewatering of cellulosic pulps in a paper making machine, a dryingmachine or the like assembly. The invention also relates to a processfor producing a dried cellulosic product from a cellulosic pulpsuspension, which process comprises dilution of said pulp with water,feeding the diluted suspension into a web forming dewatering means, anddrying to provide a desired cellulosic product, such as paper, board orpulp.

In the production of paper and board, as well as in the production ofvarious other cellulosic products, a cellulosic pulp suspension isproduced by various mechanical and chemical treatments of the cellulosicraw material.

In the production of paper or board the cellulosic pulp suspensionenters the stock preparation of a paper machine and Generally undergoesbeating, after which it is called stock. Through gradual dilution withwhite water the stock consistency is lowered to about 3-4%. To producepaper with good formation, high strength and high quality, theconsistency must be lowered further down to about 0.1-1%, which is donein the short circulation using white water straight from the wiresection.

The stock is sprayed out of the head box to a wire, where it isdewatered to a dry solids content of about 20% and then called web. Theweb is further dewatered in the press section by passing through one ormore press nips. In the drying section the web is in contact with hotdrying cylinders and the dry solids content is raised to a final 90-98%.The final paper may be rolled up on a roll, and may then be furtherprocessed in a winder or in a sheeting machine.

In a pulp mill the cellulosic pulp suspension is similarly directed tostock preparation and fed to a drying machine. The stock is generallyscreened and diluted with white water in a short circulation to aconsistency of about 0.9 to 2%, then fed to through a head box onto adewatering device such as a wire section. The dewatered web issubsequently pressed, dried and wound or cut into a final product. Thedried pulp so produced is either used as an intermediate product forpaper formation or to provide finished cellulosic products of variousforms.

Irrespective of whether the pulp suspension will form dried pulp orpaper or board, the consistency of the pulp suspension being fed to thedewatering means of the product producing assembly is very low, below 2%and often below 1%. The high water content is needed in order to producea desired end product. However, most of the water in the suspension isdrained off immediately after the head box in a dewatering means such asthe wire section of a Fourdrinier machine or a twin wire machine.

For instance, in the production of paper the stock consistency in thehead box is as low as 0.1-1% and huge amounts of water need to bedrained on the wire section. If a paper machine runs at 500 m/min, has awidth of 6 m, produces a grammage of 150 g/m² and has a stockconsistency of 0.5%, the total flow out of the head box is 90000 1/min.99.5% of that flow is water and most of that has to be removed beforethe end of the wire section. The drainage must be carried out in acontrolled manner not to affect the final cellulosic product negatively.

If the drainage is improved, the producer can make use of that by e.g.increasing the speed or decreasing the concentration in the head boxfurther. The first-mentioned alternative will increase the productionand the latter will improve the formation of the web, which is positivefor many quality parameters.

There are a number of conventional ways of improving the dewatering,e.g. by adjusting the vacuum under the wire, by using mechanical devicessuch as foils or by the addition of chemicals aids. However, there isstill a need for improving the drainage of cellulosic pulps in thedewatering means of paper and pulp mills and the present invention aimsat meeting this need.

It has now been found that treating an aqueous cellulosic pulpsuspension with carbon dioxide just prior to its entering the dewateringmeans surprisingly improves the drainage of the aqueous pulp suspensionin a significant way.

Accordingly, the present invention concerns a process for improvingdrainage of a cellulosic pulp suspension in a web forming dewateringdevice, comprising treating said pulp suspension and/or the water usedfor diluting the same with carbon dioxide just prior to said pulpsuspension entering said dewatering device, said carbon dioxide beingused in an amount sufficient to significantly improve the drainage insaid dewatering device.

The pulp suspension to be treated with the carbon dioxide according tothe invention may be any stock which has entered the short circulationof a paper making machine in a paper mill or a drying machine in a pulpmill. Said pulp may comprise virgin fibers or recycled fibers or anycombination thereof.

For the proper performance of the invention the carbon dioxide should befed into the pulp suspension at a location close to the dewateringdevice. In fact, the carbon dioxide addition point should be in theshort circulation, after any machine screening and close to the headbox.

In the preferred embodiment of the invention the carbon dioxide is fedin the form of gaseous carbon dioxide directly into the flow enteringthe head box which feeds the suspension into the dewatering device.Feeding of carbon dioxide gas in accordance with the invention into theflow at this very sensitive location of a paper machine surprisinglydoes not negatively affect the web formation on the subsequent wiresection. On the contrary, the feeding of the carbon dioxide provides animmediate and clearly visible improvement in the drainage as the dryline on the wire section, i.e. the boundary between reflecting andnon-reflecting regions of the upper surface of the fibre mat, moves backin response to the addition of carbon dioxide.

The carbon dioxide may also be introduced into the water used to dilutethe pulp suspension just prior to the head box, such as into the whitewater in the short circulation of a paper machine or a drying machine.

The amount of carbon dioxide introduced into the flow according to theinvention should not exceed the amount capable of dissolving in saidflow. The amount of carbon dioxide introduced into a pulp suspensionjust prior to a dewatering device should be about 0.5 to 5 kg CO₂/tonpaper or pulp, preferably about 1 to 3 kg CO₂/ton paper or pulp.

The carbon dioxide may be added in the form of liquid or solid carbondioxide or as carbon dioxide dissolved in a liquid. However, addition ingaseous form is considered the preferred form. Gaseous carbon dioxide iseasy to distribute uniformly into the liquid flow and it has not beenfound to provide eddies or turbulence in the flow.

The mechanism by which the carbon dioxide added in accordance with theinvention performs its beneficial action on the drainage of the waterfrom the cellulosic pulp is not known. However, the effect of the addedcarbon dioxide is clearly visible and reproducible, as described above.

Carbon dioxide has previously been used in paper making for improvingthe washing of pulp as described in EP Patent 0 296 198 (AGAAktiebolag). Here the carbon dioxide is added to washing water or in awashing device in an amount which lowers the pH of the pulp and improvesthe washing-out of substances which contribute to chemical oxygen demand(COD).

Carbon dioxide has also been used in the sizing of an aqueous pulp withalkylketene dimers to provide bicarbonate ions required to catalyze thereaction between the sizing agent and the cellulose, as disclosed in EPPatent application 0 572 304 (Canadian Liquid Air Ltd).

Finally, carbon dioxide has been used to regulate the pH of a pulpsuspension which is to be fibrilated in a refiner, as disclosed in EPPatent 0 281 273 (The BOC Group, Inc.). According to said Patent gaseouscarbon dioxide is introduced into an alkaline cellulosic pulp upstreamof the fibrilation step. The carbon dioxide feed is regulated to providea pH of 8.5 to 6.5 in the fibrilation step. The lowered pH at refiningis said to improve the physical properties of the paper and to makepossible abetter drainage of the pulp. An additional amount of carbondioxide may be introduced after refining to further lower the pH to7.0-5.5 prior to introduction into the paper making assembly.

The above prior art use of carbon dioxide does not suggest adding carbondioxide into the low consistency stock in the short circulation and itis believed that there is a strong prejudice in the art for adding a gasto the stock flow just prior to the head box. However, in the presentinvention the adding of the carbon dioxide is performed as close to thehead box as practically possible in order to have the fresh carbondioxide in the flow as it enters the wire section.

The present invention also concerns a process for producing a finalcellulosic product such as paper, board or dried pulp. According to theinvention the process comprises dilution of a pulp suspension withwater, feeding the diluted suspension into a dewatering means, anddrying to provide a desired cellulosic product, such as paper, board orpulp. The drainage of the suspension in said dewatering device isimproved by the introduction of carbon dioxide into said pulp suspensionand/or into said diluting water just prior to said pulp suspensionentering said dewatering device, said carbon dioxide being used in anamount sufficient to significantly improve the drainage in saiddewatering device.

Above, the invention has been described with reference to a typicalpaper making machine and with feeding of carbon dioxide close to thehead box. It is however, obvious to those skilled in the art that theinvention may be used also in other types of machines for producingpaper, board or pulp and that the carbon dioxide may be mixed into theflow at various other points not far from the head box.

The invention will now be further illustrated by some examples, whichare, however, not to be considered as limiting the invention in any way.

EXAMPLE 1

In a mill one board machine produced board from fully bleached kraftpulp. The machine had an air cushion head box and a conventionalFourdrinier wire section. The major grade produced on the machine was agrammage of 150 g/m².

A first trial was conducted, which involved addition of 2 kg CO₂/ton ofboard to the stock just prior to the head box. The grammage and allprocess control parameters concerning the short circulation, the headbox and the wire section were kept constant during a reference period aswell as during the trial. The addition point was located after themachine screen and before the head box.

Very shortly after the CO₂ addition was started, the position of the dryline on the wire section backed almost 20 cm due to improved dewatering.

Process parameters in the press and drying sections were adjusted toensure that the final moisture content of the board was not effectednegatively. After 2.5 hours the CO₂ addition was stopped and the dryline more or less immediately returned to its first position.

EXAMPLE 2

A second trial was performed on the same paper machine and in the samemanner as in Example 1. Once again the dry line backed when 2 kg CO₂/tonboard was added before the head box, but this time the improveddewatering was used to decrease the stock consistency in the head box.Since the speed was kept constant, the decreased consistency resulted inmore water being sprayed on to the wire and the dry line therefore movedback close to its original position. The formation of the board wasimproved.

EXAMPLE 3

In order to evaluate the effect of the CO₂ feed on the pH and the gascontent of the pulp suspension, the following test was performed:

A paper machine head box was fed a stock of bleached kraft pulp having aconsistency of 0.4% and a pH of 4.6. The machine was set to produce apaper having a grammage of 80 g/m². Carbon dioxide gas was added to theflow prior to the head box in an amount of about 2.5 kg CO₂/ton paper.

As described above, the dry line on the wire backed about 20 cm as soonas the CO₂ feeding was begun and moved to its original position when thefeeding was stopped. The pH and the gas content of the stock pulpsuspension was measured after the CO₂ addition point just before thehead box.

The pH of the flow entering the head box did not change in response tothe CO₂ feed. It remained throughout the test at about 4.6. Thus, theimproved dewatering was not due to a pH change.

Although gaseous carbon dioxide was fed into the suspension before thehead box, no gas bubbles were found in the suspension entering the headbox. Analysis of the gas content of the aqueous suspension indicatedthat all the CO₂ had dissolved.

What is claimed is:
 1. A process for improving drainage of a cellulosicpulp suspension in a web forming dewatering device, said suspensionbeing diluted with water and fed into a head box, and from the head boxto the dewatering device, characterized in that at a location close tothe head box, carbon dioxide in the form of carbon dioxide gas isdirectly introduced into said pulp suspension entering a the head box inthe short circulation of a wire section, said carbon dioxide beingintroduced in an amount sufficient to provide fresh carbon dioxide as itenters the dewatering device to significantly improve drainage in saiddewatering device.
 2. The process according to claim 1, characterized inthat said dewatering device comprises a wire section of a paper machineor a drying machine.
 3. The process according to claim 1, characterizedin that said carbon dioxide is introduced in an amount of 0.5 to 5.0kg/ton paper or pulp.
 4. The process according to claim 3, characterizedin that said carbon dioxide gases introduced into a paper making stockhaving a consistency of about 0.1 to 1.0%.
 5. The process according toclaim 3, characterized in that said carbon dioxide gas is introducedinto a pulp stock having a consistency of about 0.9 to 2.0%.
 6. Theprocess according to claim 1, characterized in that said carbon dioxideis introduced in an amount of 1 to 3 kg/ton of paper or pulp.
 7. Theprocess according to claim 1, characterized in that said carbon dioxideis introduced as the last treatment before said head box.
 8. A processfor producing paper, board or dried pulp from an aqueous cellulosic pulpsuspension which is diluted with water and fed into a head box, and fromthe head box fed to a dewatering device for providing a web which ispassed on through pressing and drying to a final product, characterizedin that at a location close to the head box, carbon dioxide in the formof carbon dioxide gas is directly introduced into said pulp suspensionentering the head box in the short circulation of a wire section, saidcarbon dioxide gas being introduced in an amount sufficient to providefresh carbon dioxide in the flow as it enters the dewatering device tosignificantly improve the drainage in said dewatering device.
 9. Processaccording to claim 8, characterized in that said carbon dioxide isintroduced into said pulp suspension immediately prior to a head boxfeeding said suspension to said dewatering device.
 10. Process accordingto claims 8 or 9, characterized in that said carbon dioxide isintroduced in an amount of 0.5 to 5 kg/ton paper or pulp.
 11. Processaccording to claims 8 or 9, characterized in that said carbon dioxide isintroduced in an amount of 1 to 3 kg/ton paper or pulp.
 12. The processaccording to claim 1 or 8, characterized in that said carbon dioxide isintroduced in an amount which does not exceed the amount capable ofdissolving in said pulp suspension.